Objective-ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats-13) cleaves von Willebrand factor, thereby modulating thrombosis and inflammation. Low plasma ADAMTS13 activity is associated with cardiovascular events, including myocardial and cerebral infarction. Here, we investigated the role of ADAMTS13 in the development of early atherosclerosis in a murine model. Methods and Results-Apolipoprotein E-null (ApoE -/-) and Adamts13-null (Adamts13 -/-) ApoE -/- mice were fed with a high-fat Western diet for 12 weeks. Atherosclerotic lesions in the aorta and aortic roots were quantified after staining. Leukocyte rolling and adhesion onto cremaster venules after oxidative injury were determined by intravital microscopy. Although plasma cholesterol levels were largely similar in both groups, the extent of atherosclerotic lesions in the aorta en face and in the aortic roots in the Adamts13 -/-ApoE -/- mice increased ≈5.5-fold (P=0.0017) and ≈6.1-fold (P=0.0037), respectively. In addition, the ratio of plasma high-to low-molecular-weight von Willebrand factor multimers increased ≈3-fold. The leukocyte rolling velocities were significantly reduced (P<0.001), with an increased number of leukocyte rolling (P=0.0026) and macrophage infiltration into the atherosclerotic lesions in the Adamts13 -/-ApoE -/- mice. Conclusion-Our results suggest that Adamts13 -/- plays a critical role in modulating the development of early atherosclerosis, likely through the proteolytic cleavage of ultra-large von Willebrand factor multimers, thereby inhibiting platelet deposition and inflammation. © 2012 American Heart Association, Inc.